Control of end-point forces of a multijoint limb by functional neuromuscular stimulation

A multivariable feedback controller was designed and tested for regulating the magnitude and orientation of the force vector at the end point of a multijoint limb in contact with an isometric load. The force vector was produced by electrical stimulation of muscles. Two coupling issues must be dealt with by the control system. First, there is a geometric coupling between the end-point force vector and joint torques on which amplitude and orientation of the force vector depend. Second, torques at two joints may be coupled due to activation of muscles that cross them (biarticular coupling). To eliminate the geometric coupling, a transformation of controller error from the Cartesian space to the joint space was employed. A multivariable proportional-plus-integral (PI) control law was used to calculate muscle activation based on the transformed controller error. Centralized and decentralized controls were investigated for decoupling the effects of biarticular muscles. Test results show that the magnitude and orientation of end-point forces could be regulated by this controller.